In present day machine tools, electric drives are used for moving various machine tools in a controlled manner. Thus, the axis drives ensure that the individual axes of the machine tool are moved into the required positions. Spindle drives provide for the rotary motion of a tool, such as of a milling tool or drill. To this end, a numerical control converts motions specified in a parts program into nominal position values that are transmitted to a control unit that includes a position controller, speed controller and current regulator. In the current regulator, nominal voltage values for individual motor phases are generated that are converted by a frequency converter using what is generally referred to as pulse width modulation.
A frequency converter draws electrical energy from a multiphase alternating current grid in that a direct voltage is first generated by a rectifier which is used to charge a DC link capacitor to a desired DC link voltage. An inverter connects the phases of a motor via two switching elements to the positive and negative DC link voltage in each case. The switches are appropriately switched in accordance with the pulse width modulation (PWM) pattern, in order to generate the required voltage potential at each phase of a motor that is required for reaching the nominal current values in the motor. The actual motor currents are measured at the output of the frequency converter and are signaled back to the current regulator.
Various methods are known in the related art for testing the proper functioning of a frequency converter and, in the event of a fault, for localizing the fault source.
Thus, the German Examined Accepted Specification DE 10148740 B4 describes a charging circuit for the DC link of a frequency converter that is able to detect a short circuit in the motor. The design of a frequency converter known from the related art including a rectifier, a DC link and an inverter is illustrated here in FIG. 1, for example, together with the power supplying grid and the connected motor.
The European Examined Patent Application EP 1355163 B1 describes a method and a device for testing the proper functioning of a frequency converter. To this end, when the DC link is charged, PWM test patterns are output to the switching elements of the inverter. The voltage occurring at the motor connections is recorded by capacitive sensors and compared to an expected voltage pattern. Even if this method makes do with just one single test line that is capacitively coupled to the motor connections, and with a corresponding voltmeter, there is a certain additional outlay that is to be expended for installing these components, however.
A type of fault that often needs to be examined is the question of whether a motor that is at a standstill, in spite of appropriate control, is to be attributed to the absence of DC link voltage, or whether another fault is present, such as a motor that is not properly connected. Since the DC link voltage in a frequency converter typically resides within a range of several hundred volts, a direct measurement is costly. Safety considerations normally necessitate a galvanic isolation from the downstream evaluation electronics.